The recent discovery of spectrin in a wide variety of nonerythroid cells and the possibility that nonerythroid spectrin may function in a manner analogous to erythroid spectrin is of potential significance for embryonic development because spectrin is a major determinant of cell shape and deformability, cell movement and the distribution of membrane proteins. The aim of this study is to characterize an Alpha spectrin-like protein which has been identified in the preimplantation mouse embryo with respect to its macromolecular and structural properties. Calmodulin-binding ability and peptide mapping will determine its homology to other spectrins and analysis of the synthetic pattern during development will indicate the time of onset of synthesis and assembly. The ultrastructural organization of spectrin and its relationship to actin and myosin in the cortical cytoskeleton will be characterized during the time of fertilization, compaction and trophoblast outgrowth. The results of these experiments will enhance our understanding of the basic mechanisms of development and will contribute to our understanding of spectrin abnormalities.